Apparatus for redirecting optical signals in free space

ABSTRACT

Apparatus for redirecting a plurality of optical beams emanating from a respective plurality of optical transmitters arranged in a pattern along a first dimension and a second dimension. The apparatus comprises a plurality of refractive regions. Each refractive region intercepts the optical beams emanating from an associated group of optical transmitters occupying a common position in the first dimension. In addition, each particular refractive region imparts to the intercepted optical beams an angular deflection in the first dimension, the angular deflection in the first dimension being a function of the common position in the first dimension occupied by the optical transmitters from which emanate the optical beams intercepted by the particular refractive region. Use of the refractive regions reduces the amount of available deflection area left unused when a parallel set of port cards is employed for switching optical signals.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is related in subject matter to two U.S. patentapplication Ser. No. ______ entitled “APPARATUS FOR SWITCHING OPTICALSIGNALS” and “SYSTEM AND METHOD FOR CONTROLLING DEFLECTION OF OPTICALBEAMS”, both to Alan Graves, both filed on the same date as the presentapplication and both hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to optical communications and,more particularly, to an apparatus for redirecting optical signals infree space.

BACKGROUND OF THE INVENTION

As optical signals used in optical communications carry ever increasingdata rates according to an ever widening variety of data standards, itbecomes desirable to provide switching at the photonic level, i.e.,without resorting to electronic circuitry for converting the opticalsignals into the electrical domain before switching is performed. Thesetypes of optical switches are referred to as photonic (or OOO—short for“Optical Input, Optically Switched, Optical Output”) switches.

The desirable characteristics of a photonic switch are scalability,robustness and the ability to provide non-blocking performance in acompact low-cost package. Generally speaking, first-generation photonicswitches afford at most two of these benefits at the expense of theother(s) in packages compromised in size and cost due to the complex,usually fiber-guided, interconnect between the various modules of theswitch.

For example, first-generation photonic switches that are scalable byvirtue of a modular design (e.g., multiple planes on a per-wavelength,or per-wavelength-group, basis) typically require a wavelengthconversion unit to provide a satisfactory level of residual blockingperformance. This introduces inefficiencies in provisioning the switch.Also, since optical signals are converted into the electrical domain forthe purposes of wavelength conversion, switches of this type lose thedesignation of being truly photonic in nature. Moreover, in lambda-planeswitches, the optical interconnect requires up to thousands ofindividual optical fiber connections, which can be reduced in sizesomewhat by the provision of an orthogonal shuffle function, but thisnevertheless results in a non-compact solution.

Other designs, such as multi-stage photonic switches (e.g., CLOS), canbe made non-1 blocking through dilation or path rearranging, but do notscale well to accommodate an increase in the number of input signals. Inparticular, the complexity of the interconnect between stages becomesintractable as the number of input signals increases. Furthermore, inaddition to introducing a delay, the multi-stage characteristic of theseswitches imparts a higher path loss due to multiple lossy switchingoperations in series that need to be compensated for in the design.

Still other first-generation photonic switch architectures, such as theXros X-1000, utilize opposing arrays of independently controllablemirrors at the end of an optical chamber to achieve non-blockingperformance. However, these switches tend to be large in size, have lowtolerance to manufacturing error and also do not scale well due to alack of modularity. In addition, such switches have a complexfiber-based interconnect.

Against this background, it is clear that there exists a need in theindustry for improvement in the area of photonic switches.

SUMMARY OF THE INVENTION

In accordance with a first broad aspect, the present invention seeks toprovide an apparatus for redirecting a plurality of optical beamsemanating from a respective plurality of optical transmitters arrangedin a pattern along a first dimension and a second dimension. Theapparatus comprises a plurality of refractive regions. Each refractiveregion intercepts the optical beams emanating from an associated groupof optical transmitters occupying a common position in the firstdimension. In addition, each particular refractive region imparts to theintercepted optical beams an angular deflection in the first dimension,the angular deflection in the first dimension being a function of thecommon position in the first dimension occupied by the opticaltransmitters from which emanate the optical beams intercepted by theparticular refractive region.

In accordance with a second broad aspect, the present invention seeks toprovide an apparatus for switching optical signals. The apparatuscomprises a transmit entity adapted to emit a plurality of optical beamsalong a first plurality of parallel planes of travel, the parallelplanes of travel in the first plurality of parallel planes of traveloccupying respective first positions along a normal to the firstplurality of parallel planes of travel. The apparatus further comprisesa deflection entity adapted to receive the optical beams from thetransmit entity and to deflect the received optical beams into aplurality of deflected optical beams along a second plurality ofparallel planes of travel, the parallel planes of travel in the secondplurality of parallel planes of travel occupying respective secondpositions along said normal to the first plurality of parallel planes oftravel, each of the second positions being distinct from each of thefirst positions. Finally, the apparatus comprises a receive entityadapted to receive the deflected optical beams from the deflectionentity.

In accordance with a third broad aspect, the present invention seeks toprovide an apparatus for switching optical signals. The apparatuscomprises a transmit entity adapted to emit a plurality of optical beamshaving respective directions of travel. The apparatus also comprises areceive entity adapted to receive a plurality of deflected optical beamsfrom respective directions of arrival. Finally, the apparatus comprisesa reflective entity comprising a first reflective surface and a secondreflective surface held in a fixed relative position to one another, thefirst reflective surface adapted to deflect the optical beams uponreceipt from the transmit entity towards the second reflective surface,the second reflective surface being adapted to deflect the optical beamsreceived from the first reflective surface towards the receive entity asthe plurality of deflected optical beams.

These and other aspects and features of the present invention will nowbecome apparent to those of ordinary skill in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing the use of test cards for anout-of-service calibration procedure;

FIGS. 2A-2C are, respectively, perspective, side elevational and planviews of an apparatus for switching optical signals in accordance withan embodiment of the present invention;

FIG. 2D is a side elevational view of a port card for use in anapparatus for switching optical signals in accordance with an embodimentof the present invention;

FIGS. 3A-3C are, respectively, perspective, side elevational and planviews of an apparatus for switching optical signals in accordance withan embodiment of the present invention, additionally comprising a prismplate;

FIG. 3D is a perspective view of a dual faceted prism plate inaccordance with an embodiment of the present invention;

FIG. 3E is side elevational view of an apparatus for switching opticalsignals in accordance with an embodiment of the present invention,comprising a pair of reflective surfaces;

FIGS. 4A-4C are, respectively, perspective, side elevational and planviews of an apparatus for switching optical signals in accordance withan embodiment of the present invention;

FIG. 4D is a plan view of a switch for optical signals in accordancewith an embodiment of the present invention, comprising a reflector witha pair of reflective surfaces;

FIG. 5A is a plan view of an apparatus for switching optical signals inaccordance with an embodiment of the present invention, comprising areflector with a planar surface;

FIG. 5B is a plan view of an apparatus for switching optical signals inaccordance with an embodiment of the present invention, comprising areflector with a curved surface;

FIG. 5C is a plan view of an apparatus for switching optical signals inaccordance with an embodiment of the present invention, comprising areflector with multiple facets;

FIG. 5D is a more detailed depiction of the reflector of FIG. 5C;

FIG. 6 is a perspective view of a beam steering element capable ofcausing controllable deflection of an optical beam;

FIG. 7A is a block diagram of a transmit beam steering element array inaccordance with an embodiment of the present invention;

FIG. 7B is a side elevational view of an arrangement of beam steeringelements in the transmit beam steering element array of FIG. 7A;

FIGS. 7C and 7D are variants of FIG. 7B;

FIG. 7E is a plan view of an arrangement of beam steering elements inthe transmit beam steering element array of FIG. 7A;

FIGS. 7F and 7G are variants of FIG. 7E;

FIG. 8A is a flowchart illustrating operation of a control moduleresponsible for controlling the beam steering element array of FIG. 7A;

FIGS. 8B and 8C depict possible lookup table structures for use by thecontrol module;

FIG. 9A is a view of a transmit port card and a receive port card fromthe perspective of a reflector, illustrating misalignment of an opticalbeam sent in an unpredictable direction of departure;

FIG. 9B illustrates misalignment of an optical beam arriving at a beamsteering element in an unpredictable direction of arrival;

FIG. 9C is a view of a transmit port card and a receive port card fromthe perspective of a reflector, illustrating precession of an opticalbeam under control of the transmit port card;

FIG. 10 shows, in block diagram form, a circuit for detectingcharacteristics of a received optical beam;

FIG. 11 is a flowchart illustrating operation of a control moduleresponsible for executing a fine tuning process to steer an opticalbeam;

FIG. 12 illustrates the fine tuning process at various stages ofexecution.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference to FIGS. 2A-2C, an apparatus for photonically switchingoptical signals (i.e., the signals remain in a photonic form throughoutthe switch node paths) in accordance with an embodiment of the presentinvention, hereinafter referred to as a photonic switch 100, includes aprovisionable plurality of port cards 102A, 102B, also sometimesreferred to as tributary cards, trib cards, input/output (I/O) cards,etc. From a mechanical standpoint, the port cards 102A, 102B of theswitch 100 may stand on edge in a side-by-side manner, supported byslots of a card cage. The port cards 102A, 102B exchange optical signalswith other elements of the overall photonic network of which the switch100 is a part (not shown, but designated by the reference numeral 104)and which are external to the switch 100. The port cards includetransmit port cards 102A (so called because they cause optical signalsto be transmitted into an optical chamber 118) and receive port cards102B (so called because they receive optical signals from the opticalchamber 118). In some embodiments of the present invention (includingbut not limited to FIGS. 2A-2D and FIGS. 3A-3E), the port cards are dualfunction port cards, i.e., the transmit port cards 102A are the same asthe receive port cards 102B. In other embodiments of the presentinvention (including but not limited to FIGS. 4A-4C), the transmit portcards 102A are distinct from the receive port cards 102B.

The transmit port cards 102A receive input optical signals, e.g., alongfiber optic cables 106 and connectors 172, from the external entities104. Various optical processing functions are performed in customizedsignal conditioning and processing functions of the transmit port cards102A. For example, in FIGS. 2A and 2B, the input optical signal is aninput multi-carrier optical signal such as a dense wavelength divisionmultiplexed (DWDM) signal. Here, an input signal conditioning module 108on each transmit port card 102A provides demultiplexing and otherprocessing of the input optical signals. Other functionalities arepossible, dependent upon the signal conditioning and processingfunctions needing to be implemented on the transmit port card 102A. Instill other cases, for example, in FIG. 2D, where individual opticalcarrier signals are received via a ribbon cable, there is no need for aninput signal conditioning module.

The output of the input signal conditioning module 108 is a set ofindividual optical carrier signals sent to a set of respective opticalcoupling elements (such as rod lenses or “GRIN” lenses, hereinafterreferred to as optical transmitter elements 110) to couple from awaveguide environment within the substrate of the transmit port card102A into a free-space parallel sided optical beam. Thus, the opticaltransmitter elements 110 on each transmit port card 102A transform theindividual optical carrier signals from their guided wave environment onthe transmit port card 102A into respective parallel (non-divergent)optical beams 112. In an example implementation, the optical transmitterelements 110 comprise rod lenses or beam collimators aligned to thesubstrate waveguides of the transmit port card 102A by the use ofV-grooves etched into the edge of a silicon substrate, into which therod lenses or beam collimators are placed.

Each of the optical beams 112 acquires an initial direction given by thecorresponding optical transmitter element 110, which is independentlyand individually modified by one or more beam steering elements in atransmit beam steering element array 114. The resulting optical beams,hereinafter referred to as “oriented” optical beams 116, are projectedinto a free-space optical chamber 118, in the general direction of areflector 120 although at distinct and precisely controlled individualangular directions of departure, each aimed at the virtual image (in thereflector 120) of a target receiver element on one of the receive portcards 102B. The direction of each beam in three-dimensional space willhave a horizontal component (denoted by a horizontal deflection angle α,see FIGS. 2A, 2C) and a vertical component (denoted by a verticaldeflection angle η, see FIGS. 2A, 2B) for each oriented optical beam 116emanating from a given transmit port card 102A. Each of the orientedoptical beams 116 then undergoes reflection by the reflector 120 and isreceived at one of the received port cards 102B. It should be understoodthat the terms “horizontal” and “vertical” are chosen for convenienceonly, in order to describe two orthogonal dimensions, but these termsshould not be considered as restrictive.

In the embodiment shown in FIGS. 2A-2C, the optical beams received at agiven receive port card 102B, hereinafter referred to as “receivedoptical beams” and denoted by the numeral 122, impinge upon a receivebeam steering element array 140 on the given receive port card 102B. Thereceive beam steering element array 140 redirects the received opticalbeams 122 into respective deflected optical beams 142. The receive beamsteering element array 140 provides a controllable amount of deflectionwhich causes each of the deflected optical beams 142 to impinge directlyon a respective one of a plurality of optical receive elements 124,allowing those elements to focus the beams 142 accurately on to thewaveguide interface into the receive port card substrate at the far endof those elements.

In an example embodiment, the optical receive elements 124 can beconstructed similarly to the optical transmitter elements 110, e.g., asV-grooves etched into the edge of a silicon substrate which carries theoptical waveguides connecting to the rest of the receive port card 102Bin combination with rod lenses mounted in those grooves. Each rod lenshas the end of a respective waveguide at its focal point for the casewhere a parallel optical beam is input into the lens from free space ina direction along its axis. The optical receive elements 124 transformthe deflected optical beams 142 into switched optical carrier signals,which are provided in a guided wave environment to an output signalconditioning module 126. The output of the signal conditioning module126 is a plurality of switched optical signals, which are provided tothe external entities 104, e.g., along fiber optic cables 128 via aconnector 174. In an example, the output signal conditioning module 126may perform multiplexing of multiple single-carrier optical signals. Ofcourse, the output signal conditioning module 126 may perform otheroptical processing functions as required. In still other cases, forexample in FIG. 2D, where a ribbon cable connects the optical receiveelements 124 directly to the connector 174, there is no need for anoutput optical signal conditioning module.

When the transmit port cards 102A and the receive port cards 102B are ofthe type shown in FIGS. 2A-2C, the switch 100 operates as a linearlyprovisionable lambda plane switch. When the transmit port cards 102A andthe receive port cards 102B are of the type shown in FIG. 2D, the switch100 operates as a linearly scalable non-blocking switch. A mix of thetwo types of port cards allows one to build a lambda plane switch withany amount of add-drop to a lambda converter, allowing a wide variety ofswitch configurations to be constructed on a common platform. It isnoted that these advantages are enabled by placing the optical switchelements (particularly the transmit and receive beam steering elementarrays 114, 140) on the port cards, which can be done in a smallphysical space using hybrid optical integrated circuits (HOICs).Specifically, with the advent of hybrid optical integration, complexoptical functions can be monolithically integrated into asilica-on-silicon substrate (e.g., array waveguide multiplexers, opticalattenuators, thermo-optic switches, and even, with Erbium doping of thesilica, optical amplification). These functions can be further augmentedby hybridized components such as lasers, detectors and electronic chipsin order to achieve a relatively complex, electrically controlledoptical functionality in a relatively small space, especially in“height” (the dimension orthogonal to the plane of the substrate of theport card), which translates into a reduction of the required inter-cardspacing or pitch. This allows the realization of switchingimplementations, modularities and partitionings that may have beenviewed as impractical in the past.

In operation, the photonic switch 100 achieves switching action byvirtue of the deflection angles (α and η) acquired by each of theoriented optical beams 116 under the action of the transmit beamsteering element array 114 (the oriented optical beams 116 being pointedat virtual images of respective receive elements 124), and also byvirtue of the action of the reflector 120. Alternatively, the opticalreceiver elements 124 themselves can be placed on the opposite side ofthe optical chamber 118 and the optical transmitter elements 110 thentarget the oriented optical beams 116 on the optical receiver elements124 rather than on virtual images of the receivers in the reflector 120,thereby allowing the reflector to be dispensed with. In any event, byprecisely controlling the angle at which the oriented optical beams 116are sent away from the transmit port cards 102A, individual candidatebeam steering elements within each receive beam steering element array140 and the associated optical receive elements 124 can be reached asdesired.

Control of the individual beam steering elements of the transmit andreceive beam steering element arrays 114, 140 on each transmit andreceive port card 102A, 102B is effected by a control module 130responsible for the port card in question. It should be noted that thecontrol module 130 responsible for a given transmit or receive port card102A, 102B can be located on that port card itself, on another port cardor on a separate “controller card”; alternatively, the various controlmodules 130 can be consolidated onto a smaller number of separatecontroller cards.

The control module 130 receives switching instructions from a switchcontroller 134, which can be implemented as a central shared resourcethat receives and acts on connection requests by interacting with thecontrol modules 130 responsible for the various transmit and receiveport cards 102A, 102B. One non-limiting way of supplying the switchinginstructions to the control module 130 is by way of a shared data bus138. Other configurations are possible, including but not limited to adaisy chain among the port cards. The switching instructions identifyindividual combinations of optical transmitter elements 110 and opticalreceive elements 124 that are intended to be optically connected to oneanother, in order to satisfy some higher level switching function. Forexample, the switching instructions sent onto the data bus 138 mayindicate “connect the A^(th) optical transmitter element 110 on theB^(th) transmit port card 102A to the C^(th) optical receive element 124on the D^(th) receive port card 102B”. These switching instructions aresent to the control modules 130 on both the B^(th) transmit port card102A and the D^(th) port cards 102B. On the B^(th) transmit port card102A, the switching instructions are used to control the transmit beamsteering element array 114 via a link 136 on the B^(th) transmit portcard 102A, while on the D^(th) receive port card 102B, the switchinginstructions are used to control the receive beam steering element array140 via the link 136 on the D^(th) receive port card 102B.

From the above, it will be appreciated that the switching actionprovided by the switch 100 is non-blocking, since there is nothing toprevent any optical transmitter element 110 from optically connecting toany optical receive element 124 via their associated beam steeringelement arrays 114, 140 and the reflector 120. Also, it should beappreciated that as the number of transmit or receive port cards 102A,102B is increased, the capacity of the switch 100 will grow in a linearfashion in proportion to the number of additional optical transmitterelements 110 and/or optical receive elements 124 located on the addedport cards 102A, 102B. As an aside, it will be recognized that thenumber of port cards 102A, 102B, as well as the number of opticaltransmitter elements 110 per transmit port card 102A and the number ofoptical receive elements 124 per receive port card 102B, can have a widerange of values while remaining within the scope of the presentinvention.

From FIG. 2C, it will be apparent that the horizontal deflection angle αfor a given transmit port card 102A ranges from a minimum horizontaldeflection angle α_(MIN) to a maximum horizontal deflection angleα_(MAX), where the range depends on the position of the transmit portcard 102A within the card cage. For example, the optical transmitterelements 110 located on transmit port cards 102A at the center (in thehorizontal direction) have a range of potential horizontal deflectionangles α that is symmetric about zero, while the optical transmitterelements 110 located on transmit port cards 102A at the rightmost edgehave a range of potential horizontal deflection angles α that isentirely to the left, and the optical transmitter elements 110 locatedon transmit port cards 102A at the leftmost edge have a range ofpotential horizontal deflection angles α that is entirely to the right.

Now, in a design where all transmit port cards 102A are intended to beidentical, one will need to pre-design them to provide a range ofpotential deflection angles α that is greater than necessary, since oneneeds to account for the positive, symmetric and negative casesdescribed above and illustrated in FIG. 2C. The result is that, in use,each transmit port card 102A is effectively left unable to exploit abouthalf of the pre-designed range of potential horizontal deflection anglesα. This may be problematic, depending on the technology chosen forfabricating the individual beam steering elements of the transmit andreceive beam steering element arrays 114, 140. For example, consider thecase where the beam steering elements are 2-axis gimbaled MEMS mirrorshaving achievable deflection angles of +/−5-7 degrees of mechanicalmovement, resulting in +/−10-14 degrees of optical deflection. This canresult in the requirement for a deep free-space optical chamber 118 andresultant long optical paths, with the commensurate difficulties inachieving the requisite pointing accuracy, as well as holding thatpointing accuracy in the presence of mechanical vibration.

Fortunately, it is possible to reduce this ineffective use of potentialrange of horizontal deflection angle. Specifically, as shown in FIGS.3A-3C, an apparatus hereinafter referred to as a “prism plate” 300 canbe introduced between the transmit port cards 102A and the reflector120. The prism plate 300 has a number of refractive facetted verticalstrips 302, each associated with a different one of the transmit portcards 102A. The vertical strips 302 may be coated with ananti-reflective material (or multiple layers of anti-reflectivematerials, each layer being of the order of (2n+1)/4 wavelengths thickat the center of the optical frequency band of interest, where n is aninteger, usually 0, 1 or 2 but not limited to those values) covering thewavelengths of interest, such as (but not limited to) 1500 nm to 1600 nmor a subset thereof. Each of the vertical strips 302 presents a facehaving an angle relative to the general horizontal direction, which is afunction of the position (along the horizontal direction) of theassociated transmit port card 102A, in addition to being a function ofthe refractive index of the material of the prism plate 300, thephysical geometry of the reflector 120 (planar mirror or otherwise), thetotal number of transmit port cards 102A and the pitch, i.e., thespacing between the transmit port cards 102A. This will translate into aright or left bias Δα for each given vertical strip 302 that depends onthe horizontal position of the transmit port card 102A associated withthe given vertical strip 302. More specifically, the transmitterelements 110 can be viewed as defining a two-dimensional array, i.e., inthe horizontal and vertical directions. The transmitter elements 110 ona given transmit port card 102A share the same horizontal position. Eachvertical strip 302 will thus provide the same horizontal bias for theoptical beams 112 emitted by the transmitter elements 110 sharing thesame horizontal position, i.e., which are on the same transmit port card102A.

It will thus be appreciated that with the use of the prism plate 300, itis not necessary to over-provision the beam steering elements of thetransmit beam steering element array 114 on the various transmit portcards 102A to provide a larger-than-necessary range of potentialhorizontal deflection angles α. Rather, the available range of potentialhorizontal deflection angles α will always be directed towards theoptical chamber 118 by the prism plate 300. This has the advantage ofallowing a reduction in both the optical path length and the depth ofthe free-space optical chamber 118, as well as allowing a reduction inthe required pointing precision for the oriented optical beams 116emanating from the transmit beam steering element array 114 to impingeon the desired beam steering element of the receive beam steeringelement array 140.

With reference now to FIG. 2B, it will also be apparent that thevertical deflection angle η may range from a minimum vertical deflectionangle η_(MIN) (when a northernmost optical transmitter element 110 sendsan oriented optical beam 116 to a southernmost receiver element 110 onany receive port card 102B) to a maximum vertical deflection angleη_(MAX) close to zero (when a southernmost optical transmitter element110 sends an oriented optical beam 116 to a northernmost receiverelement 110 on any receive port card 102B). In fact, each of the opticaltransmitter elements 110 on a given transmit port card 102A has its ownrange of potential vertical deflection angles η, delending on thevertical position of each optical transmitter element 110 on the giventransmit port card 102A, although there is no dependence on thehorizontal position of the given transmit port card 102A within the cardcage.

Because there is no dependency of the range of potential verticaldeflection angles η on the horizontal position of a given transmit portcard 102A in the card cage, it may be of advantage to bias each opticaltransmitter element 110 “downwards” at all times, so as to pointgenerally towards the image of a optical receive element 124 somewherein the lower half of the switch 100. This will translate into a downwardvertical bias for each optical transmitter element 110 that depends onthe relative vertical position of that optical transmitter element 110.This downward vertical bias can be achieved in a variety of ways, someof which are now described.

In a first example, the downward vertical bias can be achieved by thecontrol module 130 providing a bias drive voltage to the beam steeringelements in the transmit beam steering element array 114. The bias drivevoltage can be such that the optical beam 112 emanating from eachoptical transmitter element 110 is steered via the reflector 120 towardsan existing or fictitious beam steering element corresponding to anoptical receive element 124 that is located midway between the uppermostand lowermost optical receive elements 124. The bias drive voltage isthen varied differentially (i.e., increased or reduced slightly) duringactual operation so as to point to an actual beam steering elementcorresponding to the target optical receive element 124 specific in theswitching instructions. However, this solution has the detrimentalside-effect of eroding the useful deflection range of the beam steeringelements (typically MEMS switch mirrors with +/−5-7 degrees ofmechanical movement) in a manner similar to that described before.

Alternatively, as shown in FIG. 3D, the desired downward vertical biascan be achieved by providing refraction at the output of the opticaltransmitter elements 110. Specifically, a modified prism plate 300′(which already provides the requisite horizontal bias, described above)additionally introduces a variable vertical bias, being downward for theupper half of the shelf and, depending on operation requirements, upwardfor the lower half of the shelf. This can be achieved by providingvertical strips (providing horizontal bias) on one surface of the prismplate and horizontal prism facets (providing vertical bias) on the othersurface. In an alternative embodiment to the one illustrated in FIG. 3D,the same side of the prism plate provides both a horizontal bias and avertical bias. In yet another embodiment, two prism plates could beplaced in series, one providing the horizontal bias and one providingthe vertical bias. Alternatively, instead of the prism plates beingimplemented as rows of horizontal and vertical prisms, more complexstructures and facet angles with both varying horizontal and verticalcomponents could be used on one or both surfaces, creating atwo-dimensional array of angled prism facets on each surface of theprism plate. This would allow for an increased level of deflection fromthe prism plate.

In yet another embodiment, shown in FIG. 3E, the desired vertical biascan be provided by splitting the reflector 120 into a pair of planarmirrors 320, 322 that act in a “periscope” fashion. In a specificembodiment, the reflective surfaces of the planar mirrors 320, 322 maybe perpendicular to one another. This setup further helps to reduce thedepth of the optical chamber 118. Also, this embodiment is particularlyadvantageous where there is a pre-determined constant relationshipbetween the vertical positions of all optical transmitter elements 110and all optical receive elements 124, i.e., when switching occurs onlyin the horizontal direction. For example, such a constraint may be ineffect when different beams of received monochromatic light are beingdemultiplexed and re-multiplexed at the port cards. In such cases, thereceive port cards 102B may be designed such that an optical receiveelement 124 can only receive same-colored light which means light fromoptical transmitter elements 110 occupying a common position in thevertical direction on any given transmit port card 102A. The use ofplanar mirrors 320, 322 effectively results in an inversion in the orderin which colors are distributed in a vertical direction, between theoptical transmitter elements 110 on one hand and the optical receiveelements 124 on the other. Under these circumstances, the backplanemirror periscope structure of FIG. 3E provides the requisite verticaltranslation.

In the embodiments of FIGS. 2A-2C and FIGS. 3A-3E described above, eachof the port cards 102A, 102B possesses both transmit and receivefunctionality. However, when the transmit port cards 102A are distinctfrom the receive port cards 102B, then the transmit port cards 102A andthe receive port cards 102B can be interleaved while in other specificembodiments, for example with reference to FIGS. 4A-4C, the transmitport cards 102A are located generally towards one side (in this case theleftmost side of the card cage) and the receive port cards 102B can belocated generally towards the other side. It is also noted that when thetransmit port cards 102A are distinct from the receive port cards 102Bthen, as best shown in FIG. 4D, the transmit port cards 102A and thereceive port cards 102B can be separated from one another by a one ormore other cards 420 or empty slots, which can be used for controlpurposes or future expansion.

As before, a horizontal deflection angle α and a vertical deflectionangle η for each oriented optical beam 116 emanating from a particulartransmit port card 102A is provided by the beam steering elements of thecorresponding transmit beam steering element array 114 on that transmitport card 102A. The oriented optical beam 116 then reflects off of thereflector 120 towards the appropriate receiver 124 on the appropriatereceive port card 102B via the appropriate beam steering element of thereceive beam steering element array 140 on that receive port card 102B.

From FIG. 4C, it will be apparent that the horizontal deflection angle αmay range from a minimum horizontal deflection angle α_(MIN) (when aoptical transmitter element 110 on the leftmost of the transmit portcards 102A sends an oriented optical beam 116 to the rightmost of thereceive port cards 102B) to a maximum horizontal deflection angleα_(MAX) close to zero (when an optical transmitter element 110 on therightmost one of the transmit port cards 102A sends an oriented opticalbeam 116 to the leftmost of the receive port cards 102B). In fact, eachof the port cards 102A has its own range of potential horizonaldeflection angles α, which will be different for transmit port cards102A occupying different slots in the card cage.

In a design where the transmit port cards 102A are designed to beinterchangeable, all of the transmit port cards 102A would ideally tohave the same capabilities of deflection. Therefore, in the design ofFIGS. 4A-4C, where all transmit port cards 102A are identical and wherethe range of potential horizontal deflection angles is symmetric aboutzero and designed to account for the worst-case scenario, there will beerosion of a significant percentage of the available range of potentialhorizontal deflection angles α.

Now, recalling that with current deflection technologies such as MEMS,deflection angle is a scarce commodity, it is possible to pre-orienteach optical transmitter element 110, so as to point in a direction thatcorresponds to the image of a beam steering element on an imaginaryreceive port card located midway between the rightmost and leftmost onesof the receive port cards 102B. This will translate into a rightwardbias for the optical transmitter elements 110 on each of the transmitport cards 102A that depends on the horizontal position of that transmitport card 102A within the card cage. This rightward bias can be achievedby providing a prism plate (not shown) at the output of the transmitbeam steering element arrays 114 of the various transmit port cards102A, in a manner similar to that described above with reference to theembodiment of FIGS. 3A-3C.

As before, the use of such a prism plate allows one to foregoover-provisioning the transmit beam steering element array 114 on eachtransmit port card 102A to provide a larger-than-necessary range ofpotential horizontal deflection angles α. Moreover, due to the effect ofthe prism plate, the full range of potential horizontal deflectionangles α of all the transmit port cards 102A will remain inside theoptical chamber 118, allowing the optical path length and the chamberdepth to be reduced. The path length and the chamber depth can be evenfurther reduced by extending the prism plate to provide refraction ofthe received optical beams 122 (received via the reflector 120) towardsthe beam steering elements on the receive port cards 102B. As analternative, which allows the use of less powerful prism plates or eveneliminates the need for such prism plates, one can use a horizontalperiscope setup as shown in FIG. 4D. Specifically, a pair of planarreflective surfaces 430, 440 are provided at the back of the opticalchamber 118 are serve to provide a horizontal bias to the orientedoptical beams 116 sent by the transmit port cards 102A.

Returning now to FIG. 4B, it will also be apparent that the verticaldeflection angle η may range from a minimum vertical deflection angleη_(MIN) (when an uppermost optical transmitter element 110 sends anoriented optical beam 116 to a lowermost optical receive element 124) toa maximum vertical deflection angle η_(MAX) (when a lowermost opticaltransmitter element 110 sends an oriented optical beam 116 to anuppermost optical receive element 124). In fact, each of the opticaltransmitter elements 110 has its own range of potential verticaldeflection angles η, which will be different for optical transmitterelements 110 at different vertical positions, but will not vary amongstthe transmit port cards 102A. For example, a optical transmitter element110 located mid-way between the upper and lower extremes has a range ofpotential vertical deflection angles η that is symmetric about zero,while a lowermost optical transmitter element 110 has a range ofpotential vertical deflection angles η that is entirely upwards, and anuppermost optical transmitter element 110 has a range of potentialvertical deflection angles η that is entirely downward. In this way, itis seen that the optical transmitter elements 110 will be left unable toexploit about half of their range of potential vertical deflectionangles η.

However, it is possible to harness the unused portion of the range ofpotential vertical deflection angles η of the optical transmitterelements 110. Specifically, a second prism plate (not shown, but similarto the prism plate 300′ of FIG. 3D) can be introduced between thetransmit port cards 102A and the reflector 120. The second prism platewill have a number of refractive facetted horizontal strips (similar tothe strips 312), each associated with an optical transmitter element 110in a different position along the vertical direction. The horizontalstrips of the second prism plate may comprise one or more coatings ofanti-reflective material covering the wavelengths of interest, such as1500 nm to 1600 nm or a subset thereof. Each of the horizontal stripshas an angle relative to the general vertical direction, which is afunction of the vertical position of the associated optical transmitterelement 110, in addition to being a function of the refractive index ofthe material of the second prism plate, the physical geometry of thereflector 120 (planar mirror or otherwise), the total number of opticalreceive elements 124 and the spacing therebetween. This will translateinto a vertical bias for each of the optical transmitter elements 110that depends on the vertical position of that optical transmitterelement 110.

It will thus be appreciated that with the use of the second prism plate,the full range of potential vertical deflection angles η of all theoptical transmitter elements 110 will be utilized, allowing the opticalpath length and the chamber depth to be reduced. Also, it should benoted that the first and second prism plates can be placed one in frontof the other, or they can be integrated to form a single composite prismplate, similar to the prism plate 300′ of FIG. 3D, but adapted toaccount for the new geometry which separates the transmit port cards102A from the receive port cards 102B.

The reflector 120 is now described in greater detail. The configurationof the reflector 120 has an influence on the depth of the opticalchamber 118 as well as on the precise direction in which the transmitbeam steering element array 114 must send the oriented optical beams 116in order for them to reach their intended optical receive element 124,as specified in the switching instructions. For example, the completeabsence of a reflector is one possibility, where the transmit port cards102A and the receive port cards 102B face one another at opposite endsof a optical chamber 118. However, the depth of the optical chamber 118is greater than in the presence of a reflector 120.

When a reflector 120 is used, such may be planar or non-planar innature. With reference to FIG. 5A, there is shown a planar mirror 502 inplan view. The beam steering elements of the receive beam steeringelement arrays 140 are associated with virtual images that are “behind”the planar mirror 502, and represent the points towards which the beamsteering elements of each transmit beam steering element array 114should aim when attempting to reach an actual target beam steeringelement. The target beam steering element and its image are equally farfrom the planar mirror 502, and are of the same size.

Now with reference to FIG. 5B, consider the case of a convex mirror 504,smaller in size than the planar mirror 502, placed at the back of theoptical chamber 118 in place of the original planar mirror 502. The useof the convex mirror 504 gives rise to a smaller virtual image, which islocated closer to the convex mirror 504 than the originating object, theratio of image magnification and front/back distances being equal.Hence, for a magnification of S (where S<1), the ratio of front-backdistances is S:1 and the ratio of the size of the image vis-à-vis theoriginal is S:1. It can thus be shown that the ratio of the overall pathlength to the image is (1+S):2 when compared with the case of the planarmirror 502, which means that the distance to the convex mirror 504 canbe reduced by a factor of (2*S/(1+S)):1. Alternatively, if the distanceto the convex mirror 504 is kept constant, then the arctangent of thisratio represents the available reduction in the total horizontaldeflection angle range, although it is noted that stronger horizontalbias by a prism plate would be needed with this approach.

One side-effect from the convex mirror 504 approach of FIG. 5B, is thatthe curvature of the surface of the convex mirror 504 will cause anastigmatic distortion, leading to an expansion (dispersion) of theoptical beams 122 leaving the convex mirror 504. This can be overcome bythe solution in FIG. 5C, which shows an alternative to both the planarmirror 502 and the convex mirror 504, namely the use of a facettedbackplane mirror 506. Specifically, the facetted backplane mirror 506comprises planar vertical facets 512 that have increasingly acute anglesas the horizontal distance from the center of the facetted backplanemirror 506 increases. In a specific embodiment, the number of planarvertical facets 512 used to support a number (P) of port cards acting asboth transmit and receive port cards is 2P−1. The facets can be designedso as to lie in a flat plane at a specific points determined by themagnification factor (S) required, but with the facet angles matchingthose of a curved mirror giving the same value of S. This gives the samebenefit of the convex mirror 504, namely a smaller deflection angle or areduced chamber depth. However, because the facets 512 are planar, therewill be no distortion of the received optical beams 122, provided thateach oriented optical beam 116 impinges upon only one of the facets 512at any given time.

An example of how to design the facetted backplane 506 for a desiredimage/object ratio of S (which is equal to H/G or J/K) is now described.The 2P−1 facets 512 are denoted 512 ₁, 512 ₂, . . . , 512 _(2P-1), whilethe P port cards are denoted 102 ₁, 102 ₂, . . . , 102 _(P). Facet 512 ₁interconnects only one port card to itself, namely 102 ₁. Facet 512 ₂interconnects two port cards, namely port card 102 ₁ and port cards 102₂. Facet 512 ₃ intercepts port card 102 ₂ to itself, as well as port 102₁ to 102 ₃. This pattern continues, until one reaches the central (i.e.,P^(th)) facet 512 _(P), which intercepts some connections from all portcards. Beyond this point, the number of port cards interconnecteddecreases until, at facet 512 _(2P-2), where just port cards 102 _(P-1)and 102 _(P). In the case of a switch 100 with eight (P=8) port cards,as is shown in FIG. 5C, this leads to: Facet # Port Cards Interconnected512₁ 102₁

102₁ 512₂ 102₁

102₂ 512₃ 102₁

102₃, 102₂

102₂ 512₄ 102₁

102₄, 102₂

102₃ 512₅ 102₁

102₅, 102₂

102₄, 102₃

102₃ 512₆ 102₁

102₆, 102₂

102₅, 102₃

102₄ 512₇ 102₁

102₇, 102₂

102₆, 102₃

102₅, 102₄

102₄ 512₈ 102₁

102₈, 102₂

102₇, 102₃

102₆, 102₄

102₅ 512₉ 102₂

102₈, 102₇

102₃, 102₆

102₄, 102₅

102₅ 512₁₀ 102₃

102₈, 102₇

102₄, 102₆

102₅ 512₁₁ 102₄

102₈, 102₇

102₅, 102₆

102₆ 512₁₂ 102₅

102₈, 102₇

102₆ 512₁₃ 102₆

102₈, 102₇

102₇ 512₁₄ 102₇

102₈ 512₁₅ 102₈

102₈

In accordance with the above, and as can be seen from FIG. 5C, everysecond facet returns light from a port card back to that port card.Thus, facet 512 ₁ is in a plane perpendicular to a line from port card102 ₁ to the center of facet 512 ₁, facet 512 ₂ is in a planeperpendicular to the bisect of the angle formed at that facet betweenlines from port cards 102 ₁ and 102 ₂, facet 512 ₃ is in a planeperpendicular to a line from port card 102 ₂ to the center of facet 512₃, facet 512 ₄ is in a plane perpendicular to the bisect of the angleformed at that facet between lines from port cards 102 ₁ and 102 ₄, etc.

As previously mentioned, the transmit beam steering element array 114 ona given transmit port card 102A is responsible for deflecting theoptical beams 112 into oriented optical beams 116, causing the latter toacquire a desired direction towards the reflector 120 (if used). It isnoted that the optical beams 112 deflected by the transmit beam steeringelement array 114 are closely spaced and arrive in parallel at thetransmit beam steering element array 114 from the optical transmitterelements 110. FIG. 7A shows generally how deflection is achieved whileFIGS. 7B through 7G show various embodiments of the transmit beamsteering element array 114. As can be appreciated from FIG. 7A and thedescription to follow, a common feature of each of configurations inFIGS. 7B through 7G is that a plurality of points of deflection areprovided for each of the optical beams 112. It will be appreciated thata similar design can be used in the receive beam steering element array140 on each receive port card 102B. It is recalled that the receive beamsteering element array 140 redirects the received optical beams 122 intodeflected optical beams 142 that impinge on the optical receive elements124.

With specific reference to FIG. 7B, there is shown a portion of a firstexample embodiment of the transmit beam steering element array 114 on aparticular transmit port card 102A, with the substrate of the transmitport card 102A being in the plane of the page. Specifically, the opticaltransmitter elements 110 produce a plurality of optical beams 112 whichimpinge on a column of beam steering elements 702, 704, 706. The beamsteering elements 702, 704, 706 each comprise a respective first,movable reflective facet 702 ₁, 704 ₁, 706 ₁. The beam steering elements702, 704, 706 also each comprise a respective second, non-movablereflective facet 702 ₂, 704 ₂, 706 ₂, which may be provided in aspecific non-limiting example embodiment by a reflectively coated backwall of an enclosure. Contrary to the first reflective facet 702 ₁, 704₁, 706 ₁ of each of the beam steering elements 702, 704, 706, which hasa controllable deflection angle, the deflection angle of the secondreflective facets 702 ₂, 704 ₂, 706 ₂ is fixed. Of course, it should beunderstood that the second reflective facets 702 ₂, 704 ₂, 706 ₂ may beprovided as stand-alone mirrors not having any connection to the firstreflective facets 702 ₁, 704 ₁, 706 ₁.

As previously described, the transmit beam steering element array 114provides at least two points of deflection for each of the optical beams112, as emitted by the optical transmitter elements 110 on theparticular transmit port card 102A of interest. In the specificembodiment of FIG. 7B, the second reflective facet 702 ₂ of beamsteering element 702 is fixed in a position where it intercepts theoptical beam 112 emitted by a corresponding one of the opticaltransmitter elements 110 and deflects it towards the first reflectivefacet 704, of beam steering element 704. Similarly, the secondreflective facet 7042 of beam steering element 704 is fixed in aposition where it intercepts the optical beam 112 emitted by another oneof the optical transmitter elements 110 and deflects it towards thefirst reflective facet 706 ₁ of beam steering element 706. Beam steeringin two axes (vertical deviations from the horizontal direction in theplane of the drawing and perpendicular to the drawing) is provided bythe first reflective facets 702 ₁, 704 ₁, 706 ₁, which in fact deliverthe second of two points of deflection for each of the resultant opticalbeams 112, resulting in the oriented optical beams 116.

With reference now to FIG. 7C, there is shown a portion of a secondexample embodiment of a transmit beam steering element array 114 on aparticular transmit port card 102A, again with the substrate of thetransmit port card 102A being in the plane of the page. This embodimentis similar to the one in FIG. 7B, except that the beam steering elements702, 704, 706 are inverted. Thus, the transmit beam steering elementarray 114 continues to provide at least two points of deflection foreach of the optical beams 112 emitted by the optical transmitterelements 110 on the transmit port card 102A. However, in the specificembodiment of FIG. 7C, the first reflective facet 702, of beam steeringelement 702, which has a controllable deflection angle, is positioned soas to intercept the optical beam 112 emitted by a corresponding one ofthe optical transmitter elements 110 and to controllably deflect ittowards the second reflective facet 704 ₂ of beam steering element 704.Similarly, the first reflective facet 704 ₁ of beam steering element 704intercepts the optical beam 112 emitted by another one of the opticaltransmitter elements 110 and controllably deflects it towards the secondreflective facet 706 ₂ of beam steering element 706. Beam steering isagain provided by the first reflective facets 702 ₁, 704 ₁, 706 ₁, butwhich in this case deliver the first (rather than the second) of twopoints of deflection for each of the optical beams 112, resulting in theoriented optical beams 116. While it is clear that the total range ofdeflection angles is the same in the embodiment of FIG. 7C, the secondreflective facets 702 ₂, 704 ₂, 706 ₂ in the embodiment of FIG. 7C needto provide a reflective area that is slightly larger than the reflectivearea that needs to be provided in the embodiment of FIG. 7B.

In another embodiment, each of the optical beams 112 is deflected by twoseparate beam steering elements having independently controllabledeflection angles. Specifically, having regard to FIG. 7D and again withthe substrate of the transmit port card 102A being in the plane of thepage, a back-to-back assembly is provided, whereby beam steeringelements 702 and 703 each have a respective reflective first facet 702₁, 703 ₁ and are joined by a common second facet 702 ₂, which need notbe reflective. Similarly, beam steering elements 704 and 705 each have arespective reflective first facet 704 ₁, 705 ₁ and are joined by acommon second facet 704 ₂, while beam steering elements 706 and 707 eachhave a respective reflective first facet 706 ₁, 707 ₁ and are joined bya common second facet 706 ₂. In another embodiment (not shown), theremay be a separation between the second facet of each pair ofback-to-back beam steering elements. In fact, the back facets of thevarious beam steering elements 702-707 are irrelevant to the opticalpath reflections in this particular embodiment.

Here again, the transmit beam steering element array 114 provides atleast two points of deflection for each of the optical beams 112 emittedby the optical transmitter elements 110 on the transmit port card 102A.Specifically, the first reflective facet 702 ₁, 703 ₁, 704 ₁, 705 ₁, 706₁, 707 ₁ of each of the beam steering elements 702, 703, 704, 705, 706,707 has a controllable deflection angle. Thus, the first reflectivefacet 7021 of beam steering element 702 is located such as to interceptthe optical beam 112 emitted by a corresponding one of the opticaltransmitter elements 110 and to deflect it towards the first reflectivefacet 705 ₁ of beam steering element 705. Similarly, the firstreflective facet 704 ₁ of beam steering element 704 is fixed in aposition where it intercepts the optical beam 112 emitted by another oneof the optical transmitter elements 110 and deflects it towards thefirst reflective facet 707 ₁ of beam steering element 706. Beam steeringis provided by each of the two reflective facets encountered by each ofthe optical beams 112, which affords a substantially increased totalrange of possible deflection angles, approximately doubling the maximumbeam deflection when compared with the embodiments of FIGS. 7B and 7C.

FIGS. 7B-7D were presented with the plane of the substrate of thetransmit port card 102A in the plane of the page. A further set ofsolutions is rendered possible by rotating the structures through 90degrees relative to the substrate. These solutions are shown in FIGS.7E-7G, where the transmit port card 102A is being viewed from above(i.e., plan view).

In FIGS. 7E and 7F, the beam steering element array 114 utilizes a stripmirror for at least one of the two deflections. Specifically, withreference to FIG. 7E, there is shown a plan view of an edge of aparticular transmit port card 102A, with the uppermost opticaltransmitter element 110 being visible in the drawing, and producing anoptical beam 112 underneath which there is an entire column of opticalbeams 112, effectively forming a parallel optical beam front. Theoptical beam front impinges upon a strip mirror 740, which deflects theoptical beams 112 into deflected optical beams 742. The strip mirror 740has a fixed deflection angle and may be formed of a single, monolithicpiece of material. The deflected optical beams 742 each impinge upon anindividual beam steering element 744, which has a reflective facet 746with a controllable deflection angle. Thus, the transmit beam steeringelement array 114 provides two points of deflection for each of theoptical beams 112 emitted by the optical transmitter elements 110 on thetransmit port card 102A.

FIG. 7F shows a similar setup to that of FIG. 7E, except that the rolesof the strip mirror 740 and the beam steering elements 744 have beenreversed. Thus, the reflective facets 746 of the beam steering elements744 provide the first deflection for each of the optical beams 112 inthe optical beam front, while the second deflection is provided by thestrip mirror 740. In this case, it is the angle of the first deflection,rather than the angle of the second deflection, which is controllable.

Yet another non-limiting example embodiment of the transmit beamsteering element array 114 is shown in FIG. 7G, using two columns ofindividual beam steering elements 752, 754. The beam steering elements752 comprise respective reflective facets 752 ₁, 754 ₁, which providetwo independently controllable deflection angles for each of the opticalbeams 112. The combination of the two independently controllabledeflective surfaces approximately doubles the achievable deflectionangle, with a commensurate shortening of required optical path length,relative to the examples with single controlled deflection surfaces.However it also requires that the area of the second deflective surfacebe enlarged slightly.

The beam steering elements in the above-described examples of thetransmit beam steering element array 114 can be implemented in manyways, one of which is now described with reference to FIG. 6. Forconvenience, the various beam steering elements, which took on referencenumerals 702, 703, 704, 705, 706, 707, 752, 744, 752 and 754, will behereinafter referred to under the numeral 600 in FIG. 6. The basicstructure of the beam steering element 600 described below is similar totechnologies such as part number ADN59102 or part number ADN59210available from Analog Devices, Norwood, Mass., USA. However, otherembodiments of the beam steering element 600 are possible, in whichdifferent mechanisms are used.

In this example implementation, not to be considered a limitation butrather an example of what can be achieved using readily availabletechnologies, the beam steering element 600 comprises a 3-D MEMS mirror602 linked to a housing via two sets of torsion members 604, 606 (forthe X and Y directions, respectively). A set of four (4) quadrantelectrodes 608 on a nearby substrate 610 underlies the back surface (notshown) of the mirror 602. The electrodes 608, which may be implementedas plates under the surface of the mirror 602, are driven withelectrostatic drive voltages to cause the mirror 602 to move to adesired position in three-dimensional space against the tension of thetorsion bar springs 604 linking the mirror 602 to the annulus and of thetorsion bar springs 606 linking the annulus to the mirror surround.Specifically, the mirror 602 is activated by placing analog controlvoltages on each of the four electrodes 608 and exploiting electrostaticattraction to point the mirror 602 in a desired direction.

While it may be advantageous to have the substrate 610 close to themirror 602 in order to achieve adequate deflection sensitivity withoutthe use of inordinately high voltages, this proximity also limits thedegree of deflection achievable with the mirror 602 before electrostaticattraction overcomes the torsion springs and the mirror “snaps-down” tomake contact with the underlying electrode 608. In current designs“snap-down” (whereby the electrostatic attraction overpowers the torsionof the torsion bar spring in a non-linear manner) can occur beyond 5-7degrees of mechanical deflection, by which point the drive voltages maybe approaching 150 volts, although it is envisaged that in futuredesigns, the range of deflection may be greater due to the use ofimproved mechanisms for steering the mirror 602.

As has been previously mentioned, the beam elements 600 in the transmitand receive beam steering element arrays 114, 140 on the transmit andreceive port cards 102A, 102B are controlled by the control module 130for the port card of interest, in response to switching instructionsreceived from the switch controller 134. Assume that the switchinginstructions require the A^(th) optical transmitter element 110 of theB^(th) transmit port card 102A to emit an oriented optical beam 116 withthe aim of eventually reaching the C^(th) optical receive element 124 ofthe D^(th) receive port card 102B (via the reflector 120, if any). Theswitching instructions are interpreted differently by the control module130 on the B^(th) transmit port card 102A and the control module 130 onthe D^(th) receive port card 102B. Specifically, the control module 130on the B^(th) port card interpets the switching instructions as “connectthe A^(th) optical transmitter element 110 to the C^(th) optical receiveelement 124 of the D^(th) receive port card 102B”, whereas the controlmodule 130 on the D^(th) receive port card 102B interprets the switchinginstructions as “connect the C^(th) optical receive element 124 to theA^(th) optical transmitter element 110 of the B^(th) transmit port card102A”. The instructions to the B^(th) transmit port card 102A ensurethat the correct optical transmitter element 110 shines in the correctdirection, while the instructions to the D^(th) receive port card 102Bensure that the correct optical receive element 124 looks in the correctdirection for incoming light. It is noted that the B^(th) transmit portcard 102A and the D^(th) receive port card 102B may in fact be the sameport card.

Reference is now made to FIG. 8A, which shows the basic steps executedby the control module 130 responsible for the B^(th) transmit port card102A upon receipt of the switching instructions. At step 810, thecontrol module 130 responsible for the B^(th) transmit port card 102Aidentifies the particular beam steering element in the transmit beamsteering element array 114 responsible for providing a controllabledeflection angle for the optical beam 112 emanating from the A^(th)optical transmitter element 110. In addition, at step 820, the controlmodule 130 responsible for the B^(th) port card determines the X and Ydrive voltages for that particular beam steering element, with theintent of establishing an optical path to the C^(th) optical receiveelement 124 on the D^(th) receive port card 102B.

A similar process is carried out for the receive beam steering elementarray 140 on the D^(th) port card. Specifically, at step 810, thecontrol module 130 responsible for the D^(th) receive port card 102Bidentifies the particular beam steering element in the receive beamsteering element array 140 responsible for shining a beam into the rodlens of the C^(th) optical receive element 142. In addition, at step820, the control module 130 responsible for the D^(th) port carddetermines the X and Y drive voltages for that particular beam steeringelement, with the intent of parallelizing a received optical beam 122picked up in the direction from the A^(th) optical transmit element 110on the B^(th) transmit port card 102A.

Of course, it should be appreciated that if more than one beam steeringelement with a controllable deflection angle is used to deflect theoptical beam 112 emanating from the A^(th) optical transmitter element110 on the B^(th) transmit port card 102A (or if more than one beamsteering element with a controllable deflection angle is used to deflectthe resultant received optical beam 122 at the receive beam steeringelement array 140), then step 810 would consist of identifying theseplural beam steering elements and step 820 would consist of obtainingthe X and Y drive voltages for each of these plural beam steeringelements. However, for the sake of simplicity but without intending tolimit the scope of the invention, it is hereinafter assumed that onlyone beam steering element in each of the transmit beam steering elementarray 114 and the receive beam steering element array 140 needs to becontrolled for any given connection.

The control module 130 on either the B^(th) transmit port card 102A orthe D^(th) receive port card 102B can perform step 820 in many ways.Consider the control module 130 on the B^(th) transmit port card 102Afor the sake of example. In one embodiment, step 820 will be performedby consulting a first lookup table (at step 822) followed by a secondlookup table (at step 824). With reference to FIG. 8B, the first lookuptable 850 maps each combination of optical transmitter element 110 onthe B^(th) transmit port card 102A and possible optical receive element124 (on any receive port card 102B) to the required X and Y angulardeflections for the beam steering element in the path of the opticalbeam 112 emanating from the optical transmitter element 110 in thecombination. The second lookup table 860 (see FIG. 8C) maps the angulardeflection per applied millivolt, for both the X and Y directions, foreach beam steering element on the B^(th) transmit port card 102A. Thus,consultation of the first lookup table 850 at step 822 results inobtaining the requisite angular deflection for the beam steering elementin the transmit beam steering element array 114 located in the path ofthe A^(th) optical transmitter element 110, whereas consultation of thesecond lookup table 860 at step 824 results in obtaining the drivevoltages necessary for achieving the requisite angular deflection. Asimilar set of tables is used on the D^(th) port card to establish thedrive voltages to the beam steering elements in the beam steeringelement array 140 so that they couple a beam from the appropriatedirection into the optical receive elements 124 on the D^(th) port card.

The first lookup table 850 can be populated analytically from thephysical geometry of the switch 100, i.e., based on parameters such asthe depth of the optical chamber 118, the spacing between the port cards(i.e., pitch), as well as the presence or absence of a prism plate 300(and its refractive characteristics, if present). Since the transmitport cards 102A are interchangeable, it may be advantageous to store thefirst lookup table 850 in volatile memory to allow modification as theswitch 100 is scaled, although this is not a requirement.

The second lookup table 860, i.e., which maps angular deflection toapplied voltage for the beam steering elements on a given transmit portcard 102A (e.g., the B^(th) transmit port card 102A), can be populatedduring an initialization phase of the manufacturing process of the giventransmit port card 102A. By way of example, this initialization phasemay entail pointing each beam steering element of the given transmitport card 102A at a variety of test detectors in order to compute a“deflection sensitivity map” for the beam steering element. In oneembodiment, this may require a large number of values for both the X, Ydirections for each beam steering element. In another embodiment, asmaller number of vertical and horizontal locations is established,while the rest are computed by a polynomial “form-fit”. The voltagesrequired to achieve specific deflections, whether obtained directly orthrough polynomial interpolation, form the second lookup table 860.Since the second lookup table 860 is specific to the hardware on thegiven transmit port card 102A, it may be advantageous to store thesecond lookup table 860 in non-volatile memory, although this is not arequirement.

As an alternative to maintaining the two lookup tables 850, 860, asingle composite lookup table could be created and stored in volatilememory, thus (in the case of the B^(th) transmit port card 102A, forexample) mapping each combination of optical transmitter element 110 (onthe B^(th) transmit port card 102A) and possible optical receive element124 (on any receive port card 102B) to the required X and Y voltages tobe applied to the beam steering element in the path of the optical beam112 emanating from the optical transmitter element 110 in thecombination. Yet another alternative would be to fit a very high orderpolynomial to the values in such composite lookup table and to store thecoefficients of the resultant polynomial. In this way, a polynomialcomputation is required on the part of the control module 130, therewill be a reduced need for memory, since only the coefficients of thepolynomial need to be stored.

Regardless of the manner in which step 820 is performed, the result willbe that (i) the oriented optical beam 116 resulting from action of thetransmit beam steering element array 114 upon the optical beam 112emanating from the A^(th) optical transmitter element 110 will be shonetowards the reflector 120 in a direction that is intended to cause thereceived optical beam 122 to reach the C^(th) optical receive element124 on the D^(th) receive port card 102B; and (ii) the beam steeringelement array associated with the C^(th) optical receive element 124 onthe D^(th) receive port card 102B will capture an incoming optical beam122 from the direction associated with the A^(th) optical transmitterelement 110 on the B^(th) transmit port card 102A and couple it into theC^(th) optical receive element 124. With, say, a +/−7 degree full-scaledeflection and a one-in-10,000 resolution, the use of the look up tables850, 860 allows aiming of a particular beam steering element to aprecision of about 0.7 milli-degrees, which, at the end of an opticalpath that may be of the order of a meter in length, results in an“aiming granularity” on the order of roughly 0.24 mm, i.e., the locationof the end of the received optical beam 122 in three-dimensional spacecan be controlled with an initial pointing precision of 0.24millimeters.

It should be noted that due to a variety of factors, one might notalways be able to rely on the beam steering elements producing correctlyaligned oriented optical beams 116 based on pre-computed lookup tablesor polynomials. In other words, the above-defined pointing precisiondoes not necessarily translate into a pointing accuracy. For instance,while it is possible to produce changes as small as 0.24 mm in thevertical or horizontal location of the received optical beams 122, theinitial oriented optical beam 116 may be misaligned to begin with, thisdespite the manufacturing calibration performed to produce the secondlookup table 860. Examples of possible error sources in obtainingconsistent pointing accuracy include:

-   -   repeatability of the setting of individual beam steering        elements (e.g., MEMS mirrors 602); although it is expected to be        excellent, there might be an unknown aging mechanism in the        mirror deflection torsion members;    -   tolerances in the X, Y and Z positions occupied by individual        transmit and receive port cards 102A, 102B as they are held in        position by the slots of the card cage;    -   tolerances in the angular positions occupied by individual        transmit and receive port cards 102A, 102B as they are held in        position by the slots of the card cage;    -   errors in the angles of the strips 302 of the prism plate 300;        with a refractive index of, say, 1.5, a one-degree facet angle        will produce about 0.3-0.5 degree of pointing error, depending        on the angle of incidence and other factors; based on what is        commercially available for precision prisms it is reasonable to        control facet angles (by precision grinding) to +/−0.01 degree        or better giving rise to approximately 0.003-0.005 degrees of        pointing error;    -   errors in the flatness of the reflector 120 and its angular        positioning at the end of the optical chamber 118; assuming that        the reflector 120 can be made optically flat, the main error        will be the depth of the optical chamber 118, which may have        approximately 0.5 mm of depth error.

Assuming a 28 degree optical deflection cone (i.e., +/−7 degreesmechanical movement in each of the X and Y directions) and a path lengthof 1 meter, a tally of the worst-case error from the above sources mayresemble the following: digitization resolution/presets: +/−0.12 mm cardslot tolerance in X, Y, Z dimensions:  +/−0.2 mm card slot tolerance(angular): +/−0.17 mm prism facet angle: +/−0.09 mm reflector placement:+/−0.53 mm TOTAL +/−1.11 mm

With a mirror having dimensions of roughly 1 mm in diameter, the aboveworst-case cumulative error is sufficient for the received optical beam122 to miss the target beam steering element in the receive beamsteering element array 140.

Now, using some example dimensions not indicative of any limitation orrestriction of the present invention, if the pitch of the port cards is7.5 mm and the spacing between adjacent optical transmitter elements 110is greater than about 1.5 mm, then the use of the lookup tables 850 and860 will orient the beam steering element in the transmit beam steeringelement array 114 so that the ensuing received optical beam 122 pointssomewhere in an imaginary circle of diameter 2.2 mm, centered on thetarget beam steering element in the receive beam steering element array140.

With reference to FIG. 9A, this imaginary circle, hereinafter referredto as a “circle of uncertainty” 900, surrounds an “area ofdetectability” 910 representative of the available detection area of thetarget beam steering element in the receive beam steering element array140. Assuming that the width of the received optical beam 122 at the endof its optical path to be 650 microns, it becomes apparent that althoughthe received optical beam 122 might not be pointing directly towards thearea of detectability 910, it is nonetheless “close by”, i.e., somewherein the surrounding circle of uncertainty 900. Thus, the first challengeis to control the appropriate beam steering element in the transmit beamsteering element array 114 so as to cause the received optical beam 122to point directly at the area of detectability 910, i.e., towards thecenter of the circle of uncertainty 900.

Still, even with the received optical beam 122 pointing directly at thearea of detectability 910, it is possible that the target beam steeringelement in the receive beam steering element array 140 will cause anerror in deflecting the received optical beam 122 towards thecorresponding optical receive element 124 having its own “area ofdetectability”. With reference to FIG. 9B, the use of the lookup tables850 and 860 will orient the beam steering element in the receive beamsteering element array 140 so that the ensuing deflected optical beam142 will point somewhere in a circle of uncertainty 950 but notnecessarily directly at the area of detectability 952 associated withthe corresponding optical receive element 124 and whereby the opticalreceive element 124 can correctly focus the deflected optical beam 142on to the exiting waveguide into the rest of the receive port card 102B.Thus, the second challenge is to control the appropriate beam steeringelement in the receive beam steering element array 140 so as to causethe deflected optical beam 142 to impinge directly on the appropriateoptical receive element 124. It is noted that the circle of uncertainty950 is somewhat smaller than the circle of uncertainty 900 due to theshorter distance between the beam steering element and the opticalreceive element 124.

In order to shine the oriented optical beam 116 of interest onto thetarget beam steering element in the receive beam steering element array140, or in order to shine the received optical beam 122 onto thecorresponding optical receive element 124, the control module 130 on theappropriate transmit or receive port card 102A, 102B performs a “finetuning process”, which is optional. In other words, it should beunderstood that the discussion to follow is merely illustrative of anexample way to improve the pointing accuracy when such improvement isdesired, and in no way implies the necessity to improve the pointingaccuracy. Depending on the quality and tolerances of the components ofthe switch 100, it may or may not be sought to improve the pointingaccuracy afforded by straightforward execution of step 820 in thecontrol module 130 of both the transmit port card 102A and the receiveport card 102B.

Expressed in general terms, the fine tuning process solves the problemof locating an area of detectability (e.g., 910, 952) from somewhere ina surrounding circle of uncertainty (e.g., 900, 950). To this end, thecontroller 130 on the transmit port card 102A causes a controlled andvariable level of sinusoidal modulation voltages (tones) to be added inphase quadrature to the X and Y drive voltages applied to the beamsteering element in the transmit beam steering element array 114 whichemits the oriented optical beam 116 of interest. Similarly, thecontroller 130 on the receive port card 102B causes a controlled andvariable level of sinusoidal modulation voltages (tones) to be added inphase quadrature to the X and Y drive voltages applied to the beamsteering element in the receive beam steering element array 140 whichdeflects the received optical beam 122 of interest towards thecorresponding optical receive element 124.

In the case of each or either beam steering element being implemented asa mirror 602 (see FIG. 6), the modulation voltages are applied bysinusoidally varying the voltages applied to the electrodes 608responsible for movement in the +X and +Y directions, but at oppositephases between the +X, −X electrodes and between the +Y and −Yelectrodes, and in phase-quadrature between the +X, +Y electrodes. Asshown in FIG. 9C for example, the addition of modulation voltages inphase quadrature in the above described way causes the oriented opticalbeam 116 deflected by the mirror 602 to be driven into an angulardisplacement (“wobble” or “precession”) which sweeps an orbitaltrajectory 902 with a period corresponding to the frequency of themodulation voltages.

The amplitude of the modulation voltages are designed (or can becontrolled) to make the orbital trajectory 902 sufficiently wide so asto intersect the area of detectability 910. For ease of understanding,it will be assumed in what follows that the modulation voltages appliedto the X and Y drive voltages cause the oriented optical beam 116 toprecess at a frequency (or “precession tone”) f_(T). However, applyingdifferent modulation voltages to the X and Y drive voltages changes thetrajectory 902 and controls the precession orbit diameter, and it shouldbe understood that such modifications to the trajectory 902 are wellwithin the scope of the present invention.

A similar technique process is applied to when deflecting the receivedoptical beam 122 towards the corresponding optical receive element 124.In this case, the amplitude of the modulation voltages are designed (orcan be controlled) to make the orbital trajectory of the deflectedoptical beam 142 sufficiently wide so as to intersect the area ofdetectability 952 of the optical receive element 124. For ease ofunderstanding but without limiting the scope of the present invention,it will be assumed in what follows that the modulation voltages appliedto the X and Y drive voltages cause the deflected optical beam 952 toprecess at a frequency f_(R).

By tapping a small amount of the received optical signal into thereceive port card 102B and detecting that signal in an opto-electronicreceiver, after the optical signal has completed its transition into thewaveguide environment of the receive port card and by analyzing thefrequency, amplitude and phase of the precession tones f_(T), f_(R)present in the optical signal detected as being received at the opticalreceive element 124, and comparing these parameters to those of theprecession tone expected to be received by the optical receive element124, one can compute the “pointing error”, both in directing theoriented optical beam 116 at the transmit beam steering element array114, and in deflecting the received optical beam 122 at the receive beamsteering element array 142.

Specifically, the presence of a precession tone at frequency f_(T) inthe received optical signal indicates that the received optical beam 122is in the correct circle of uncertainty 900 to begin with, while theamplitude of the received optical signal is indicative of the radialdistance of the center of the trajectory 902 from the area ofdetectability 910, and the relative phase of the received optical signalis indicative of the angle at which the center of the trajectory 902 islocated relative to the area of detectability 910. This allowscomputation of a horizontal displacement correction dH_(T) and avertical displacement correction dV_(T) required to properly align theoriented optical beam 116.

Similarly, the presence of a precession tone at frequency f_(R) in thereceived optical signal indicates that the received optical beam 122 isin the correct circle of uncertainty 950 to begin with, while theamplitude of the received optical signal is indicative of the radialdistance of the center of the deflected optical beam 142 from the areaof detectability 952, and the relative phase of the received opticalsignal is indicative of the angle at which the center of the deflectedoptical beam 142 is located relative to the area of detectability 952.This allows computation of a horizontal displacement correction dH_(R)and a vertical displacement correction dV_(R) required to properly alignthe deflected optical beam 142.

It may be convenient to assign different sets (or ranges) of potentialvalues to f_(T) and f_(R), in order to assist in discriminating betweentransmit and receive precession tone frequencies. Furthermore, tosimplify the separation of the composite signal from the detector intothe components f_(T) and f_(R) of the resultant detected signal (whichwill contain both of the transmit and receive precession tonefrequencies), it may be convenient to use techniques including but notlimited to separating the ranges of f_(T) and f_(R) by a substantialfactor (e.g., 10:1 or more) or to use a form of orthogonal modulation ofthe f_(T), f_(R) components to simplify detectability of each in thepresence of the other.

Detection of the precession tones at frequencies f_(T) and f_(R) in theoptical signal received at the optical receive element 124 can beachieved using a circuit as shown in FIG. 10, which comprises an opticaldetector 920 connected to the output of the optical receive element 124via an optical coupler 922. The optical detector 920 may be implementedas a photodiode, while the optical coupler 922 may be implemented as afractional tap coupler. A processing unit 924 is connected to theoptical detector 922 and possibly other optical detectors associatedwith other optical receive elements 124. The processing unit 924 isshown as residing on the receive port card 102B, although it should beappreciated that the processing unit 924 associated with a given receiveport card 102B can be located on that receive port card 102B itself, onanother port card, on a separate “controller card”, or multipleprocessing units 924 can be consolidated onto a smaller number ofseparate controller cards, which may be the same controller cards thatsupport the control units 130 if these are consolidated as well.

The processing unit 924 has the role of determining the frequency,amplitude and phase of the precession tones present in the opticalsignal detected as being received at the optical receive element 124. Itis assumed that the processing unit 924 knows f_(R) and f_(T) based onthe connection map. In the manner described above, the processing unit924 computes dH_(T), dV_(T), dH_(R) and dV_(R). The values dH_(T) anddV_(T) are supplied to control module 130 responsible for the transmitbeam steering element array 114 that emits the oriented optical beam116. This can be achieved by using the same data bus 138 used to carrythe switching instructions to the various transmit port cards 102A, forexample. The values dH_(R) and dV_(R) are supplied to control module 130responsible for the receive beam steering element array 140 that emitsthe deflected optical beam 142.

In the context of the fine tuning process, the behaviour of the controlmodule 130 responsible for the transmit beam steering element array 114that emits the oriented optical beam 116 for a particular combination ofoptical transmitter element 110 and optical receive element 124 is nowdescribed with reference to the flowchart in FIG. 11. A virtuallyidentical flowchart applies to the control module 130 responsible forthe receive beam steering element array 140 that provides the deflectedoptical beam 142 to the optical receive element 124 of this combination,based on the values dH_(R) and dV_(R). For simplicity, only the processfor controlling the transmit beam steering element array 114 will bedescribed in detail, it being assumed that a person skilled in the artwill be able to modify this process and apply it to the receive beamsteering element array 140.

It will be seen that step 822 is the same as in FIG. 8, and consists ofconsulting of the first lookup table 850 to obtain the requisite angulardeflection for the beam steering element which outputs the orientedoptical beam 116. At this point, the control module 130 executes step1110, which consists of receiving the values dH_(T) and dV_(T) from theprocessing unit 924 on the receive port card 102B which houses theoptical receive element 124 of the particular combination in question.At step 1112, the control module 130 checks to see whether the finetuning process has previously been started for the particularcombination of optical transmitter element 110 and optical receiveelement 124. If not, then step 1114 is executed, where the values dH_(T)and dV_(T) are used to compute a pointing error that is compared to a“trigger threshold”. The trigger threshold 1114 is selected to representa pointing error that is sufficiently large to require the fine tuningprocess to be initiated or re-initiated. Clearly, the trigger thresholdis an arbitrary design parameter based upon the specific tolerances,dimensions and sensitivities of specific design implementations and itsselection would be a matter of routine for a person of ordinary skill inthe art.

If the pointing error is indeed greater than the trigger threshold, thecontrol module 130 executes step 1116, where the fine tuning process isformally started, followed by step 1118, by virtue of which the controlmodule 130 begins the act of monitoring the “net angular compensation”as applied (to be seen in later steps) to the X and Y angular deflectionfor the current combination of optical transmitter element 110 andoptical receive element 124. While not used right away, the value ofthis “net angular compensation” at the end of the fine tuning processwill indicate by how much the angular deflection shown in the firstlookup table 850 should have been varied in order to cause the receivedoptical beam 122 to have been shone directly onto the area ofdetectability 910.

After execution of step 1118—or if execution of step 1114 indicates thatthe pointing error was not greater than the trigger threshold, thecontrol module 130 proceeds to step 1120, where an angular compensationfor the pointing error is computed. The computed angular compensationcan be as great in absolute value as the pointing error computed fromthe values dH_(T) and dV_(T) received from the processing unit 924;however, it can be less in absolute value, so as to encourage stabilityof the feedback control loop having been created. At step 824, the X andY drive voltages are obtained from the second lookup table 860 bylooking up the angular deflection obtained at step 822 but compensatedby the value found at step 1120. Finally, as the final step in the finetuning process, step 1122 is executed, where each of the X and Y drivevoltages is modulated by a precession tone having a particular frequencyf_(T) and a particular amplitude as discussed herein below. The finetuning process subsequently returns to step 1110, where new valuesdH_(T) and dV_(T) are received from the processing unit 924. If the finetuning process is running successfully, then it is expected that thepointing error that is computed from the values dH_(T) and dV_(T)received during the next iteration of step 1110 will be no greater (inabsolute value terms) than the one during the previous iteration of step1110.

As indicated above, each of the X and Y drive voltages is modulated atstep 1122 by a precession tone, which has a particular “precessionamplitude” that should not be excessively large or exceedingly small.Specifically, it will be appreciated that when the center of thetrajectory 902 of the received optical beam 122 is far off from thecenter of the area of detectability 910, then too small a precessionamplitude will cause the trajectory 902 of the received optical beam 122to make small circles that never intersect the area of detectability910. On the other hand, too large a precession amplitude once the centerof the trajectory 902 of the received optical beam 122 has becomealigned with the center of the area of detectability 910 (i.e., after“convergence” has been achieved) will cause the trajectory 902 of thereceived optical beam 122 to make big circles that also never intersectthe area of detectability 910. For this reason, it may be advantageousduring the fine tuning process to begin with a larger amplitude beforeconvergence and to gradually decrease the amplitude of the precessiontone as convergence is achieved, and to continue doing so until it isnoticed that the pointing error has dropped to below a convergencethreshold.

The above described approach translates into additional steps in theflowchart of FIG. 11. Specifically, returning to step 1112 and assumingthat execution of this step indicates that the fine tuning process hasalready been started (i.e., due to previous execution of step 1116),then the pointing error computed at step 1110 is compared to a“convergence threshold” at step 1124. The convergence thresholdrepresents the amount of pointing error considered to be sufficientlysmall to indicate that the received optical beam 122 is satisfactorilycentered within the area of detectability 910. Clearly, the convergencethreshold is an arbitrary design parameter and its selection would be amatter of routine for a person of ordinary skill in the art.

If the pointing error is greater than the convergence threshold, thenthere continues to be a need to center the received optical beam 122within the area of detectability 910. Thus, the control module 130proceeds to step 1126, where the value of the pointing error computedduring the current iteration of step 1110 is compared to the value ofthe pointing error computed during the previous iteration of step 1110.If it is greater, then this effectively means that the received opticalbeam 122 has moved further from the center of the area of detectability910, in which case it may be desirable to increase the amplitude of theprecession tone (step 1130), so as to ensure that it will intersect thearea of detectability 910. If it is less, then this effectively meansthat the received optical beam 122 has moved closer to the center of thearea of detectability 910, in which case it may be desirable to decreasethe amplitude of the precession tone (step 1128), so as to ensure thatthe optical beam will not remain entirely outside the area ofdetectability 910 as it precesses.

Upon having decided on how to modify the amplitude of the precessiontone, steps 1120, 824 and 1122 are executed as previously described. Itshould be understood that control of the amplitude of the precessiontone (steps 1126-1130) can be effected using a more sophisticatedalgorithm, and in some cases the amplitude of the precession tone neednot be varied at all, or it may be varied differently in the X and Ydirections, or it may be varied in a manner that is independent of themagnitude of the pointing error received at step 1110.

As centering of the received optical beam 122 within the area ofdetectability 910 is achieved over time, the pointing error willeventually fall below the convergence threshold, and the “YES” branchemanating from step 1124 is taken, followed by stoppage of the finetuning process at step 1132. Next, step 1134 provides for the tallyingof the net angular compensation (computed at each execution of step1120) over the duration of the fine tuning process since it was startedat step 1116. The net angular deflection so tallied represents acorrection to the angular deflection that is currently maintained in“row” of the first lookup table 850 corresponding to the currentcombination of optical transmitter element 110 and optical receiveelement 124. Accordingly, step 1136 provides the option of modifyingthis “row” of the first lookup table 850 by the amount of the netangular compensation. By making this modification to the first lookuptable 850, the fine tuning will be accelerated in the event that thecurrent combination of optical transmitter element 110 and opticalreceive element 124 is disconnected but then needs to be re-connected ata future time. Finally, since convergence has been achieved, there is noneed to compensate the angular deflection that was initially obtained atstep 822 (i.e., step 1120 can be skipped). Also, the X and Y drivevoltages remain the same as before (i.e., step 824 can be skipped) andthe precession amplitude need not be changed (i.e., step 1122 can beskipped). The algorithm thus returns to step 1110, where new valuesdH_(T) and dV_(T) are received from the processing unit 924.

To illustrate the effects of fine tuning process, reference is had toFIG. 12, which shows corrections being applied over time (at instantsT1, T2, T3, T4) to fine tune the angular deflections of the beamsteering element producing the oriented optical beam 116. It is apparentthat the precession amplitude is reduced until, eventually, the detectedprecession tone is small enough in amplitude that the received opticalbeam 122, when precessing but locked on target, remains fully within thearea of detectability 910. In other words, at instant T4, the processingunit 924 detects a full-strength signal but no precession tone.

By continuing the precessing motion during normal operation of theswitch 100 (i.e., even after convergence), it is possible to detect ifand when the deflected optical beam 120 ceases to be fully within thearea of detectability 910. If the received optical beam 122 partly exitsthe area of detectability 910, the precessing motion of the orientedoptical beam 116 will cause the received optical beam 122 to oscillatein and out of the area of detectability 910, and this will be detectedby the processing unit 924. However, in other embodiments, the controlmodule 130 may, in response to convergence, end the fine tuning processby simply stopping the precessing motion of the received optical beam122.

It will be appreciated that when the switching instructions change, thecontrol module 130 responsible for each transmit port card 102A respondsaccordingly by re-executing the algorithm in FIG. 8 and obtaining new Xand Y drive voltages for the various beam steering elements associatedwith the optical transmitter elements 110 on the transmit port card 102Ain question. If appropriate, the fine tuning process described above andwith reference to FIG. 11 is re-initiated for each new combination ofoptical transmitter element 110 and optical receive element 124. It isnoted that if step 1136 has previously been executed for the newcombination of optical transmitter element 110 and optical receiveelement 124, then the fine tuning process will be dramaticallyshortened, since the first lookup table 850 will already bepre-compensated. Any further fine tuning will be due to equipment aginghaving occurred since the previous time the particular combination ofoptical transmitter element 110 and optical receive element 124 neededto be connected, and re-execution of the fine tuning process allows theeffects of such equipment aging to be detected, tracked and compensated.

As an alternative or an enhancement of the fine tuning process, anout-of-service calibration procedure can be used, as now described withreference to FIG. 1. Under such circumstances, one or more test cards180, 182 are provided. For example, in the case where there are two testcards 180, 182, one could be positioned in each of the leftmost andrightmost slots of the card cage. In an alternative embodiment,individual ones of the transmit or receive port cards 102A, 102B couldbe temporarily removed and replaced with a single test card. Each of thetest cards 180, 182, rather than containing beam steering elements forproviding the requisite parallelization of the received optical beams122, contains a fixed array 184 of small photodiodes (not unlike a CCDin a digital camera) which will allow for the measuring of the actuallocation and distribution of a received optical beam 122. The test card180 also comprises a control module 186 which processes the output ofthe photodiode array 184.

As part of the out-of-service calibration procedure, the A^(th) opticaltransmitter element 110 on the B^(th) transmit port card 102A isselected (and this selection is known to the control module 130 on theB^(th) port card as well as the control module 186 on the test card180), and the corresponding optical beam 112 is deflected by thetransmit beam steering element array 114 on the B^(th) port card withthe intention of reaching a chosen “C^(th) optical receive element” onone of the test cards, say test card 180. However, the “C^(th) opticalreceive element on test card 180” is imaginary because the test card 180does not contain optical receive elements but instead contains aphotodiode array 184 which might conveniently be implemented as an arraysimilar to a small CCD array as is used in digital camera technology.The photodiode array 184 detects the exact location of the receivedoptical beam 122, which may (but likely will not) correspond to theposition that would have been occupied by the beam steering elementcorresponding to the “C^(th) optical receive element” had it beenpresent. This difference in positions represents a pointing error, whichis then converted into a compensation signal, and the process isrepeated until the pointing error is sufficiently low to be consideredsatisfactory.

The above-described approach allows the major sources of tolerances(e.g., due to prism wedge angle and backplane spacing) to be compensatedfor before the first “in-traffic” connection, thereby either simplifyingthe precession routine or permitting less stringent tolerances on thebackplane mirror, prism sheet, card positioning, etc.

Those skilled in the art will appreciate that in some embodiments, thefunctionality of the control module 130 and the processing module 924may be implemented as pre-programmed hardware or firmware elements(e.g., application specific integrated circuits (ASICs), electricallyerasable programmable read-only memories (EEPROMs), etc.), or otherrelated components. In other embodiments, the control module 130 and theprocessing module 924 may be implemented as an arithmetic and logic unit(ALU) having access to a code memory (not shown) which stores programinstructions for the operation of the ALU. The program instructionscould be stored on a medium which is fixed, tangible and readabledirectly by the control module 130 and the processing module 924, (e.g.,removable diskette, CD-ROM, ROM, or fixed disk), or the programinstructions could be stored remotely but transmittable to the controlmodule 130 and the processing module 924 via a modem or other interfacedevice (e.g., a communications adapter) connected to a network over atransmission medium. The transmission medium may be either a tangiblemedium (e.g., optical or analog communications lines) or a mediumimplemented using wireless techniques (e.g., microwave, infrared orother transmission schemes).

While specific embodiments of the present invention have been describedand illustrated, it will be apparent to those skilled in the art thatnumerous modifications and variations can be made without departing fromthe scope of the invention as defined in the appended claims.

1. Apparatus for redirecting a plurality of optical beams emanating froma respective plurality of optical transmitters arranged in a patternalong a first dimension and a second dimension, comprising: a pluralityof refractive regions; each refractive region intercepting the opticalbeams emanating from an associated group of optical transmittersoccupying a common position in the first dimension; each particularrefractive region imparting to the intercepted optical beams an angulardeflection in the first dimension, the angular deflection in the firstdimension being a function of the common position in the first dimensionoccupied by the optical transmitters from which emanate the opticalbeams intercepted by the particular refractive region.
 2. Apparatusdefined in claim 1, the angular deflection in the first dimensionfurther being a function of the angle at which the optical beams arriveat the apparatus.
 3. Apparatus defined in claim 1, the refractiveregions being first refractive regions, the apparatus furthercomprising: a plurality of second refractive regions; each secondrefractive region intercepting the optical beams emanating from anassociated group of optical transmitters occupying a common position inthe second dimension; each particular second refractive region impartingto the intercepted optical beams an angular deflection in the seconddimension, the angular deflection in the second dimension being afunction of the common position in the second dimension occupied by theoptical transmitters from which emanate the optical beams intercepted bythe particular second refractive region.
 4. Apparatus defined in claim3, the angular deflection in the second dimension further being afunction of the angle at which the optical beams arrive at theapparatus.
 5. Apparatus defined in claim 4, the first refractive regionsbeing integrally formed into a first integral component, the secondrefractive regions being integrally formed into a second integralcomponent, the first and second integral components being placed inseries with one another.
 6. Apparatus defined in claim 5, each of thefirst refractive regions defining a respective strip along a surface ofthe first integral component
 7. Apparatus defined in claim 6, each ofthe second refractive regions defining a respective strip along asurface of the second integral component.
 8. Apparatus defined in claim3, the apparatus being implemented as an integral formation having afront face and a back face, wherein the first refractive regions appearon one of the front and back face and wherein the second refractiveregions appear on the other of the front and back face.
 9. Apparatusdefined in claim 1, each refractive region comprising a plurality ofrefractive sub-regions; different refractive sub-regions of the samerefractive region intercepting respective optical beams emanating fromoptical transmitters occupying different respective positions in thesecond dimension; each refractive sub-region of a particular refractiveregion imparting to the respective intercepted optical beam adisplacement in the second dimension, the displacement in the seconddimension being a function of the position in the second dimensionoccupied by the respective intercepted optical beam.
 10. Apparatus forswitching optical signals, comprising: a transmit entity adapted to emita plurality of optical beams along a first plurality of parallel planesof travel, the parallel planes of travel in the first plurality ofparallel planes of travel occupying respective first positions along anormal to the first plurality of parallel planes of travel; a deflectionentity adapted to receive the optical beams from the transmit entity andto deflect the received optical beams into a plurality of deflectedoptical beams along a second plurality of parallel planes of travel, theparallel planes of travel in the second plurality of parallel planes oftravel occupying respective second positions along said normal to thefirst plurality of parallel planes of travel, each of the secondpositions being distinct from each of the first positions; and a receiveentity adapted to receive the deflected optical beams from thedeflection entity.
 11. Apparatus defined in claim 10, wherein thedeflection entity comprises: a first reflective surface; a secondreflective surface; the first reflective surface being arranged so as todeflect the optical beams received from the transmit entity towards thesecond reflective surface; the second reflective surface being arrangedso as to deflect the optical beams received from the first reflectivesurface towards the receive entity.
 12. Apparatus defined in claim 11,wherein the first and second reflective surfaces are held in a fixedrelative position.
 13. Apparatus defined in claim 12, wherein the firstand second reflective surfaces are substantially perpendicular to oneanother.
 14. Apparatus defined in claim 13, wherein the first and secondreflective surfaces are planar mirrors.
 15. Apparatus defined in claim10, wherein the first position occupied by the plane of travel in thefirst plurality of planes of travel that carries a given optical beam,and the second position occupied by the plane of travel in the secondplurality of planes of travel that carries the given optical beam upondeflection by the deflection entity, are symmetrically disposed relativeto a point on the normal to the first plurality of parallel planes oftravel.
 16. Apparatus defined in claim 10, wherein the optical beamsoccupying a common plane of travel in the first plurality of parallelplanes of travel are non-parallel with respect to one another. 17.Apparatus defined in claim 16, wherein the optical beams occupying acommon plane of travel in the first plurality of parallel planes oftravel have a common optical carrier wavelength.
 18. Apparatus definedin claim 17, wherein the optical beams occupying a common plane oftravel in the second plurality of parallel planes of travel arenon-parallel with respect to one another.
 19. Apparatus defined in claim18, wherein the optical beams occupying a common plane of travel in thesecond plurality of parallel planes of travel have a common opticalcarrier wavelength.
 20. Apparatus for switching optical signals,comprising: a transmit entity adapted to emit a plurality of opticalbeams having respective directions of travel; a receive entity adaptedto receive a plurality of deflected optical beams from respectivedirections of arrival; a reflective entity comprising a first reflectivesurface and a second reflective surface held in a fixed relativeposition to one another, the first reflective surface adapted to deflectthe optical beams upon receipt from the transmit entity towards thesecond reflective surface, the second reflective surface being adaptedto deflect the optical beams received from the first reflective surfacetowards the receive entity as the plurality of deflected optical beams.21. Apparatus defined in claim 20, the transmit entity being adapted tocontrol the direction of travel of each of the optical beams. 22.Apparatus defined in claim 21, the first and second reflective surfacesbeing substantially perpendicular to one another.